Reference is made to commonly owned, co-pending U.S. patent applications Ser. No. 12/793,398, filed Jun. 3, 2010, Ser. No. 12/793,441, filed Jun. 3, 2010, and Ser. No. 12/793,470, filed Jun. 3, 2010.
The present disclosure relates to a discharge chamber for a compact high intensity discharge lamp, and more specifically to a compact metal halide lamp made of translucent, transparent, or substantially transparent quartz glass, hard glass, or ceramic discharge chamber materials. Compact arc discharge lamps find particular application, for example, in the automotive lighting field, although it will be appreciated that selected aspects may find application in related discharge lamp environments for general lighting encountering similar issues with regard to salt pool location and maximizing luminous flux emitted from the lamp assembly. For purposes of the present disclosure, a “discharge chamber” refers to that part of a discharge lamp where the arc discharge is running, while the term “arc tube” represents that minimal structural assembly of the discharge lamp that is required to generate light by exciting an electric arc discharge in the discharge chamber. An arc tube also contains the pinch seals with the molybdenum foils and outer leads (in the case of quartz arc tubes) or the ceramic protruded end plugs or ceramic legs with the seal glass seal portions and outer leads (in case of ceramic arc tubes) which ensure vacuum tightness of the discharge chamber plus the possibility to electrically connect the electrodes in the discharge chamber to the outside driving electrical components via the outer leads pointing out of the seal portions of the arc tube assembly.
High intensity metal halide discharge lamps produce light by ionizing a fill, such as a mixture of metal halides, mercury or its replacing buffer alternative, and an inert gas such as neon, argon, krypton or xenon or a mixture of thereof with an arc passing between two electrodes that extend in most cases at the opposite ends into a discharge chamber and energize the fill in the discharge chamber. The electrodes and the fill are sealed within the translucent, transparent, or substantially transparent discharge chamber which maintains a desired pressure of the energized fill and allows the emitted light to pass through. The fill (also known as “dose”) emits visible electromagnetic radiation (that is, light) with a desired spectral power density distribution (spectrum) in response to being vaporized and excited by the arc. For example, rare earth metal halides provide spectral power density distributions that offer a broad choice of high quality spectral properties, including color temperature, color rendering, and luminous efficacy.
In current high intensity metal halide discharge lamps, for example in automotive gas discharge lamps, a molten metal halide salt pool of overdosed quantity typically resides in a central bottom location or portion of a generally ellipsoidal or tubular discharge chamber, when the discharge chamber is disposed in a horizontal orientation during operation. Since location of the molten salt pool is always at the coldest part of the discharge chamber, this location or spot is often referred to as a “cold spot” of the discharge chamber. The overdosed molten metal halide salt pool that is in thermal equilibrium with its saturated vapor developed above the dose pool within the discharge chamber and is located inside the discharge chamber of the lamp at the cold spot, forms a thin liquid film layer on a significant portion of an inner surface of the discharge chamber wall. In this position, the dose pool distorts a spatial intensity distribution of the lamp by increasing light absorption and light scattering in directions where the dose pool is located within the discharge chamber. Moreover, the dose pool alters the color hue of light that passes through the thin liquid film of the dose pool.
Still another consideration is the impact of the electric lead wires in a lamp assembly which are for creating electrical contact between the electrodes in the discharge chamber and the electrical contacting points on the lamp base or cap. These electric lead wires of the lamp assembly can either be extended portions of the outer leads pointing out of the seal area of the arc tube assembly, or additional metal wires firmly connected to these outer leads of the arc tube assembly. In a single ended arc discharge lamp with double ended arc tube construction, one of the electric lead wires is much longer than that of the other one, and extends generally parallel all along a length of the arc tube from a proximal end to a distal end of the arc tube as seen from the lamp base in order to mechanically and electrically connect the lamp base with a distal seal portion of the arc tube. For the purposes of the present disclosure “single ended lamp” means a lamp that has a single base including both electrical contacting points of the lamp and placed at a specific single end portion of the lamp while “double ended arc tube” means an arc tube with its two electrodes located at the opposite ends of the discharge chamber. This specified distal end electric lead wire connecting to the distal end of the arc tube also has a strong shading effect on the light emitted by the arc discharge since light rays directed toward this distal end electric lead wire are either absorbed or scattered by this distal end electric lead wire. There exist arc discharge lamp constructions where this distal end electric lead wire runs outside the protective outer envelope surrounding the arc tube of the lamp and is often covered by a tube of electrically insulating material against arcing between this distal end electric lead wire and the surrounding. In such cases, degree of light blocking is exaggerated by increased effective diameter of the distal end electric lead wire due to its insulating tube cover. Because of the inevitable need to also provide the distal end electric lead wire to electrically connect the distal end of the lamp to its base, this impact of the distal end lead wire on the light output from the arc tube is usually unavoidable in known arc discharge lamps.
Optical designers who design beam forming optical systems and reflector arrangements around these types of high intensity discharge lamps that employ the described lamp, arc tube assembly and discharge chamber arrangement must recognize and accommodate both issues caused by the liquid dose pool distributed on the inner surface of discharge chamber wall and the distal end electric lead wire extending generally in parallel relation to and all along the longitudinal axis of the arc tube assembly. That is, construction of the optical system must address spatial light intensity distribution distortion, discoloration of light rays and all other light quality degradation effects in these lamps. For example, in the past and even in contemporary automotive headlamp constructions, the distorted light rays were either blocked out, by non-transparent metal shields, or the light rays were distributed evenly in directions that were not critical for the application. In other words, these distorted rays passing through the liquid dose pool were generally ignored. As such, this portion of the emitted light represents losses in the optical system as the distorted rays did not take part in forming the main beam of the projecting optical system.
In an automotive headlamp application, for example, the distorted rays are used for slightly illuminating the road immediately preceding the automotive vehicle, or the distorted light rays are directed to road sips placed well above the road. Due to these losses, efficiency of the optical systems is typically no higher than approximately 40% to 50%.
As compact discharge lamps become smaller in wattage and additionally adopt reduced geometrical dimensions, a solution is required with the light source in order to avoid such optical losses in the optical assembly or system. An improved optical system equipped with discharge lamps of improved beam characteristics would desirably achieve higher illumination levels along with lower energy consumption of the overall lighting system.
Thus, a need exists to address the issues associated with the dose pool in the discharge chamber and the distal end electric lead wire of the lamp, and their impact on performance and efficiency of the optical system designed around the lamp as a result of the uneven and distorted spatial and colorimetric light intensity distribution emitted by lamp.